In the identification of faults on a power transmission line by a protective relay which is located at a given point along the line, it is extremely important to be able to identify the direction of the fault on the transmission line relative to the location of the relay. The fault direction is either downstream of (in front of) the protective relay, which is referred to herein as a forward fault, or upstream of (in back of) the relay, which is referred to herein as a reverse fault. The ability of a protective relay to provide direction information for faults is referred to generally as its directional security. High directional security, meaning providing accurate directional information, is of great importance in the overall operational value of the relay.
Accurate directional information guarantees that the relay will never assert, i.e. provide a fault indication which results in the tripping of a circuit breaker, for faults in the reverse direction if the fault is located in the forward direction and vice versa. A relay which looks for faults in both directions will have either separate forward and reverse directional elements or a single element capable of providing information for both directions.
There are different types of directional elements, among the most popular being negative sequence polarized directional elements, and zero sequence polarized directional elements. Negative sequence directional elements are capable of declaring fault direction for all unbalanced faults. However, it is known that negative sequence elements, i.e. those elements using negative sequence quantities of voltage and current to make direction determinations, do have certain advantages over other approaches, including zero sequence elements. These advantages include insensitivity to zero sequence mutual coupling, and a generally nigher sensitivity to remote ground faults with high fault resistance.
In the conventional negative sequence element technique, the phase angle of the negative sequence voltage is compared against the phase angle of the negative sequence current. If the negative sequence current leads the negative sequence voltage, regardless of the amount of the angular difference, then the fault is in the forward direction; otherwise it is in the reverse direction.
However, there are certain disadvantages to conventional negative sequence directional elements when the negative sequence source upstream of the relay is strong, such that the impedance is low. In this situation, the negative sequence voltage at the relay will be quite low, which in turn reduces the sensitivity of the relay. It is desirable that such relays be sufficiently sensitive that they operate both for close-in faults and for remote faults, as well as operating very fast (less than one cycle) in response to the occurrence of a fault. With a strong source, the negative sequence directional element may not produce reliable results for close-in and remote faults.
In order to increase the sensitivity of the conventional negative sequence directional element in such situations, a compensating boost to the negative sequence voltage is sometimes provided, which in turn makes the phase angle measurement for close-in and remote faults easier and more reliable. However, this compensating boost in turn creates a disadvantage in certain situations, because a significant boost could result in a reverse fault looking like a forward fault to the forward directional element, a very undesirable result. In addition, it is often difficult and/or time consuming for the operator/engineer to make a proper selection of the boost.
It would be most desirable for a directional relay to have separate forward and reverse direction sensitivities, individually selectable; however, this has not been accomplished heretofore with negative sequence directional elements.
Hence, there remains a need for a directional element which is secure but also sensitive to faults on the transmission line.